The present invention relates to a particle collecting apparatus which is employed in a transfer apparatus for transferring an object to be processed afloat within a transfer passage by means of an air stream, and particularly in an air stream transfer apparatus for transferring semiconductor wafers in a semiconductor manufacturing apparatus.
In semiconductor manufacturing apparatuses, transfer apparatuses are employed in order to transfer the semiconductor wafers between the processing apparatuses. Standard transfer apparatuses of this type are transfer apparatuses in which the semiconductor wafer cassette jig is loaded on a transfer carriage, and transfer apparatuses in which the semiconductor wafers are transferred one at a time by a robot arm. However, in such transfer apparatuses, the atmosphere of the space in which the semiconductor wafers are processed is not separated from the outside atmosphere, and the atmosphere of this space is not sufficiently clean. Recently, methods have been employed in which the semiconductor wafer cassette jig is placed in a sealed container for transfer; however, the problems of contamination from the container materials and the air tight gaskets and the like has not been solved.
For this reason, air stream transfer apparatuses which transfer semiconductor wafers through a transfer passage by means of an air stream (air bearing) have been experimentally produced, and such air stream transfer apparatuses enable a closed type (closed system) transfer passage, so that it is possible to attain a sufficiently high level of cleanliness in the atmosphere.
However, in the air stream transfer apparatuses, even if the structure is such as to contain no parts which give rise to contamination within the apparatus, so that a clean space may be maintained, there may be cases in which particles which have been brought in from the outside, for example, particles which have been deposited on the semiconductor wafers, are released within the apparatus.
Such particles present a problem in that they are then deposited on other clean semiconductor wafers. Additionally, in concert with the miniaturization and increase in functionality of the semiconductor integrated circuits and the like which are formed on these semiconductor wafers, the situation is such that, increasingly, almost no contamination or deposition of matter can be tolerated, so that this problem is important in view of an increase in the reliability of the semiconductor manufacturing apparatus and in the productivity (yield) of the semiconductor wafers.
The present invention was created in order to solve the problems described above; it has as an object thereof to provide a particle removing apparatus which is capable of preventing the deposition of particles on objects to be processed within a transfer passage.
The air stream transfer apparatus of the present invention is an air stream transfer apparatus for transferring an object to be processed by means of an air stream, characterized in that a transfer face and a transfer passage partition, which are faces to be cleared of dust and which together constitute a transfer passage, are made of an electroconductive material and each have a thin insulating layer on the surface thereof; a planar dust collecting electrode is opposed to an object to be processed; a mechanism is provided for bringing the dust collecting electrode and the faces to be cleared of dust close to each other while jetting air; and a mechanism is provided for applying voltages between the transfer face and the dust collecting electrode and between the transfer passage partition and the dust collecting electrode.
It is preferable that a semiconductor material having a surface resistivity within a range of 5xc3x97105xe2x88x92108 xcexa9xc2x7cm in the shape of a plate-shaped body or a film be employed as the dust collecting electrode described above.
It is preferable that the distance between the dust collecting electrode described above and the transfer face which is to be cleared of dust, and between the dust collecting electrode and the transfer passage partition which is to be cleared of dust, be less than 2.0 mm. A distance within a range of 0.2-0.5 mm is more preferable.
It is preferable that a circuit be provided which generates an electrical field between the transfer face, the transfer passage partition, and the dust collecting electrode described above, and that a power source apparatus be provided which is capable of selectively switching between a positive and negative applied electrical field.
It is preferable that the amount of air jetted during the removal of particles be made variable, so as to maintain the insulation by adjusting the very small gap between the dust collecting electrode and transfer face and transfer passage partition by means of an air layer.
An air stream transfer apparatus of the present invention is an air stream transfer apparatus for conveying objects to be processed by means of an air stream, characterized in that a transfer face and a transfer passage partition, which are faces to be cleared of dust and which together constitute a transfer passage, are made of an electroconductive material and each has a thin insulating layer on the surface; a mechanism for transferring a precharged semiconductive plate-shaped body afloat along the transfer passage is provided, and a semiconductive plate-shaped body is used as the dust collecting electrode.
The particles which enter into the apparatus are affected by various forces, such as gravity, Brownian motion, inertial motion, static electric force, Brownian dispersion, van der Waals forces, and the like, and are deposited on the transfer face and the transfer passage partition. If the particle size is one micrometer or less, it becomes extremely difficult to remove such particles under dry processing conditions while maintaining a highly clean atmosphere. Among methods for such removal while maintaining a highly clean atmosphere, electrostatic force (Coulomb force) is the most powerful; however, experimental results show that such removal is difficult, irrespective of the particle material (good conductors such as metals or the like, semiconductors, non-conductors, or resins).
In order to remove particles using electrostatic force, it is necessary to weaken the force of adhesion in advance and to make the particles easily charged by electrostatic induction.
As a result of experimentation, it has been determined that by employing conductive materials having thin oxide film insulating layers formed thereon, and by applying an electric field to the particles on the surface of the insulating layers, removal by means of electrostatic force is facilitated. In such a case, it is thought that the various forces acting on the particles which are described above are weakened.
Examples of conductive materials having insulating layers formed thereon which are capable of maintaining a highly clean atmosphere include, for example, those in which Al2O3 is formed on the surface of highly pure aluminum or an aluminum alloy material, which avoids heavy metal contamination and involves little release of moisture or other gases, and those in which Cr2O3 is formed in a thickness within a range of 5-10 micrometers on the surface of stainless steel SUS316L. Nitrides are acceptable in place of oxides.
Among insulating materials, those with high dielectric constants are polarized by electric fields and interfere with the release of particles, and further weaken the Coulomb force, so that dielectric substances are not appropriate.
When particles are captured at the dust collecting electrode by electrostatic force, those particles having a resistivity of less than 105 xcexa9xc2x7cm lose their charge, and an electrostatic attractive force in the opposite direction acts on these particles by electrostatic induction from the electrical field and they move in the direction of the transfer face and transfer passage partition, and recharging and recapturing is repeated, so that so-called induction reentrainment occurs.
Using particles of pure aluminum (A1100), this was increasingly likely to occur as the surface resistance of the dust collecting electrode decreased, or as the particle size increased. It was determined that by using a dielectric plate having a surface resistance of 108 xcexa9xc2x7cm as the dust collecting electrode, the redispersion of conductive particles of 20 micrometers or less could be prevented. This is thought to result from a delay in the charge neutralization of the particles, or from the supportive effect of van der Waals forces once small particles have been captured.
The insulating layer type particles described above are the same as non-conducting ones irrespective of the type of material, and in order to charge these using electrostatic induction and collect them, it is effective to bring the dust collecting electrode and the transfer face and transfer passage partition into close proximity. Experimental results show that by setting the distance to 2.0 mm or less, and more preferably to within a range of 0.2 mm 0.5 mm, and by setting the applied voltage within a range of 1 kVxe2x88x922.5 V, it is possible to collect particles of one micrometer or less.
The particle collecting method of the air stream transfer apparatus of the present invention described in claim 6 will now be explained.
In the present invention, the transfer face and the transfer passage partition are made of an electroconductive material and each have a thin insulating layer on the surface thereof, and a mechanism is provided for transferring a semiconductive plate-shaped body afloat along a transfer passage formed by the transfer face and the transfer passage partition, and this semiconductive plate-shaped body is transferred through the transfer passage afloat as the dust collecting electrode. The cleaning of the interior of the apparatus may be conducted by providing the semiconductive plate-shaped body used for the dust collecting electrode with a positive or negative charge in advance with respect to the transfer face and transfer passage partition, placing this within the apparatus in place of the object to be processed, transferring the plate-shaped body afloat at a height of less than 2 mm, and preferably at a height within a range of 0.2-0.5 mm, completely capturing the particles, and recovering this electrode once it has exited the transfer apparatus.
In such a case, it is possible to reliably adsorb particles whether they are positively or negatively charged. Furthermore, by increasing the jet gas flow rate from jet hole 4, thus increasing the float height of the plate-shaped body, and conducting transfer in the vicinity of the transfer passage upper part (ceiling part) 1b, this may be cleaned, and simultaneously, the cleaning of the particles in the space within the transfer passage and floating in the air stream is conducted.
The various bodies shown in FIG. 5 may be employed as the semiconductive plate-shaped body. In FIG. 5, the white portions indicate plate-shaped bodies comprising a semiconductive material having a resistance of 5xc3x97105xe2x88x92108 xcexa9xc2x7cm, while the parts which are cross-hatched indicate ferroelectric charge parts which are formed on the plate-shaped body comprising a semiconductive material.
The dead weight and electrostatic attraction force of A-K are as given below.
A circuit which generates an electrical field between the dust collecting electrode and the transfer face and transfer passage partition, and a power source apparatus which is capable of freely switching the positive and negative polarity of the electrical field applied, are provided, and during cleaning, a voltage is applied to the apparatus, and an electrical field is generated.
During transfer of the object to be processed, the dust collecting electrode is connected to the same potential as the transfer face and transfer partition, so that electrostatic interference does not extend to the object to be processed.
During the removal of particles, the very small gap between the dust collecting electrode and the transfer face and transfer passage partition is maintained, so that by making the air stream from the jet holes variable, and by means of the air layer produced in the very small gap, the insulation can be maintained. This is particularly effective when a film is used as the dust collecting electrode.